A mobile device, such as a smartphone, tablet, etc., has a screen that allows a user to navigate web pages, play games, watch videos or perform other activities. However, conventional mobile devices have a fixed size screen such that the size of the screen cannot be varied according to the user's different activities. For example, it is more convenient to use a small screen mobile device when the user navigates web pages or plays games, and a large screen mobile device provides a better watching experience when the user watches videos. The user needs to purchase both a small screen mobile device and a large screen mobile device if the user wants to achieve the best user experience when performing different activities, which greatly increases the cost to the user and is not convenient to carry.